Acenaphthylene solvent effect

As mentioned briefly in Chapter 5, the photodimerization of acenaphthylene is subject to a very interesting heavy-atom solvent effect. The results of the photolysis of acenaphthylene in some heavy-atom solvents are given in Table 10.6.<4a) The data in Table 10.6 show that the heavy-atom solvents n-propyl bromide and ethyl iodide yield product ratios similar to that obtained in the sensitized photolysis, indicating a greater role of the triplet state in... 

It has been possible to employ the heavy-atom solvent effect in determining the rate constants for the various intercombinational nonradiative transitions in acenaphthylene and 5,6-dichIoroacenaphthylene.<436,c,rate constants, which are not accessible in light-atom solvents due to the complexity of the mechanism and the low efficiency of intersystem crossing from the first excited singlet to the first excited triplet, can be readily evaluated under the influence of heavy-atom perturbation. 

Only light PAHs (2 or 3 rings) are soluble in water at the mgL-1 level. Figure 41 shows UV spectra of acenaphthylene in water and in acetonitrile. The shape of the two spectra is similar, and no solvent effect is observed on the absorption wavelengths. Consequently, all PAH UV spectra have been acquired in acetonitrile solutions. 

The importance of light absorption to directly populate the triplet state in the presence of heavy atoms [(10.271)] and the effect of heavy atoms on the phosphorescence of acenaphthylene [(10.27m)] are easily ruled out. No new absorption bands or increased band intensities are noted in the presence of heavy atoms. Since no phosphorescence is observed, even at low temperature, there cannot be an important effect on this process. A heavy-atom effect on the partitioning of reactions (10.27g,h) and (10.27k) is eliminated by the data of Hartmann, Hartmann, and Schenck in their study of solvent effects on the photosensitized photolysis of acenaphthylene. The influence of a number of solvents was described with the use of Kirkwood-Onsager solvent parameters (an empirical method for the correlation of reaction rate with the ability of the solvent to stabilize the change in dipole moment in proceeding to the transition state). A linear plot of log(fra/u/cw) vs. the expression [(Z) — 1)/(2Z) -f- where D is the dielectric constant, p is... 

Cowan, D.O. and Drisco, R.L.E., Solvent effect of photodimerization of acenaphthylene, Tetrahedron Lett., 1255,1967. 

Table 10.3. Solvent and Concentration Effects on the Photodimerization of Acenaphthylene<41)...

The ratio of cis (27) to trans (26) dimers produced in this reaction has been found to be solvent and concentration dependent, as can be seen in Table 10.3. The data in Table 10.3 show that the cis dimer predominates in all three solvents. It is also seen that as the acenaphthylene concentration is increased, proportionally more cis dimer is formed. This effect is slightly more pronounced in cyclohexane than in benzene (see Figure 10.2) Increases in the yield of dimer with increasing concentration would of course be expected... 

A heavy-atom effect on the photocycloaddition of acenaphthylene to acrylonitrile has also been observed.<68) The effect of heavy atoms in this case is seen as an apparent increase in the quantum yield of product formation in heavy-atom solvents as opposed to cyclohexane (the time to achieve about 42% reaction in cyclohexane is greater than that required to produce the same conversion in dibromoethane by a factor of ten). An increase in the rate of acenaphthylene intersystem crossing due to heavy-atom perturbation was proposed to explain this increase in reaction rate. 

Fig. 26a and b. The effect of water content in the adsorbent on the retention volumes of standards in hexane (a) and determination of adsorbent activity a and monolayer volume (b) after reaching the critical conditions in binary solvents given in Fig. 25. Xo = 1.71 styrene, 2.02 naphthalene, 2.14 acenaphthylene, 3.02 benzopyrene-3,4 641...

The original objective in preparing emulsion polymers from the 2,6-dichlorostyrene and acenaphthylene was to obtain polymeric fillers of higher Tg than that of polystyrene. It was also presumed that these fillers would not be bonded chemically to the rubber during vulcanization and that the copolymers with butadiene would enable such bonding to be effected. Actually, the polydichlorostyrene and polyacenaphthylene did become bonded to the rubber, as indicated by the inability to extract most of the filler by solvents. The final result was that the copolymers with butadiene served merely as fillers of lower Tg than the above homo-... 

Singlet acenaphthylene (64) stereospecifically gives the syn dimer 65a, while in the T, reaction the anti dimer 65b predominates. The syn-anti ratio can be influenced by solvents with heavy-atom effect (Cowan and Drisco, 1970b), as well as by micellar solvents (Ramesh and Ramamurthy, 1984). 

Transition-state effects in which a /1-hydroxy group stabilizes the transition state for cis attack of water on a /i-hydroxycarbocation may also be responsible for the observation that cis diols are the major diols formed in the acid-catalyzed hydrolysis of acenaphthylene oxide and cydo pcn t a[cd py rene oxide.65 Due to this rateenhancing effect, /i-hydroxycarbocations may be more reactive with solvent than their parent carbocations that do not contain the -hydroxy group. 

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